Field of the Invention
The present invention relates to solid state lighting devices, including devices with narrow-band emitters stimulated by at least one solid state light emitter, and methods of making and using same.
Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
Solid state light sources may be utilized to provide colored (e.g., non-white) or white LED light (e.g., perceived as being white or near-white). White solid state emitters have been investigated as potential replacements for white incandescent lamps due to reasons including substantially increased efficiency and longevity. Longevity of solid state emitters is of particular benefit in environments where access is difficult and/or where change-out costs are extremely high.
A solid state lighting device may include, for example, at least one organic or inorganic light emitting diode (“LED”) or a laser. A solid state lighting device produces light (ultraviolet, visible, or infrared) by exciting electrons across the band gap between a conduction band and a valence band of a semiconductor active (light-emitting) layer, with the electron transition generating light at a wavelength that depends on the band gap. Thus, the color (wavelength) of the light emitted by a solid state emitter depends on the materials of the active layers thereof. Solid state light sources provide potential for very high efficiency relative to conventional incandescent or fluorescent sources, but present challenges in simultaneously achieving good efficacy, good color reproduction, and color stability (e.g., with respect to variations in operating temperature).
Color reproduction is commonly measured using Color Rendering Index (CRI) or average Color Rendering Index (CRI Ra). In the calculation of the CRI, the color appearance of 14 reflective samples is simulated when illuminated by a reference illuminant and the test source. The difference in color appearance ΔEi, for each sample, between the test and reference illumination, is computed in CIE 1964 W*U*V* uniform color space. It therefore, provides a relative measure of the shift in surface color and brightness of an object when lit by a particular lamp. The general color rendering index CRI Ra is a modified average utilizing the first eight indices, all of which have low to moderate chromatic saturation. The 9th indice (R9) may also be significant as the R9 essentially demonstrates how well a lamp shows off a strong red. This is important for warmer color temperatures, as there is much more red in the spectrum. A high R9 (greater than 0) is commonly expected in order to mimic a “warm candle light” type color. The CRI Ra equals 100 (a perfect score) if the color coordinates and relative brightness of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator. Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80) for general illumination use where the colors of objects are not important. For some general interior illumination, a CRI Ra>80 is acceptable. CRI Ra>85, and more preferably, CRI Ra>90, provides greater color quality.
CRI only evaluates color rendering, or color fidelity, and ignores other aspects of color quality, such as chromatic discrimination and observer preferences. The Color Quality Scale (CQS) developed by National Institute of Standards and Technology (NIST) is designed to incorporate these other aspects of color appearance and address many of the shortcomings of the CRI, particularly with regard to solid-state lighting. The method for calculating the CQS is based on modifications to the method used for the CRI, and utilizes a set of 15 Munsell samples having much higher chroma than the CRI indices.
Aspects relating to the present inventive subject matter may be better understood with reference to the 1931 CIE (Commission International de I'Eclairage) Chromaticity Diagram, which is well-known and readily available to those of ordinary skill in the art. The 1931 CIE Chromaticity Diagram maps out the human color perception in terms of two CIE parameters x and y. The spectral colors are distributed around the edge of the outlined space, which includes all of the hues perceived by the human eye. The boundary line represents maximum saturation for the spectral colors.
The chromaticity coordinates (i.e., color points) that lie along the blackbody locus obey Planck's equation: E(λ)=A λ−5/(eB/T−1), where E is the emission intensity, λ is the emission wavelength, T the color temperature of the blackbody, and A and B are constants. Color coordinates that lie on or near the Planckian blackbody locus (BBL) yield pleasing white light to a human observer. The 1931 CIE Diagram includes temperature listings along the blackbody locus (embodying a curved line emanating from the right corner). These temperature listings show the color path of a blackbody radiator that is caused to increase to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. This occurs because the wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature. Illuminants that produce light on or near the BBL can thus be described in terms of their color temperature.
The term “white light” or “whiteness” does not clearly cover the full range of colors along the BBL since it is apparent that a candle flame and other incandescent sources appear yellowish, i.e., not completely white. Accordingly, the color of illumination may be better defined in terms of correlated color temperature (CCT) and in terms of its proximity to the BBL. The pleasantness and quality of white illumination decreases rapidly if the chromaticity point of the illumination source deviates from the BBL by a distance of greater than 0.01 in the x, y chromaticity system. This corresponds to the distance of about 4 MacAdam ellipses, a standard employed by the lighting industry. A lighting device emitting light having color coordinates that are within 4 MacAdam step ellipses of the BBL and that has a CRI Ra>80 is generally acceptable as a white light for illumination purposes. A lighting device emitting light having color coordinates within 7 MacAdam ellipses of the BBL and that has a CRI Ra>70 is used as the minimum standards for many other white lighting devices including compact fluorescent and solid state lighting devices.
General illumination generally has a color temperature between 2,000 K and 10,000 K, with the majority of lighting devices for general illumination being between 2,700 K and 6,500 K. The white area is proximate to (i.e., within approximately 8 MacAdam ellipses of) the BBL and between 2,500 K and 10,000 K.
Because light that is perceived as white is necessarily a blend of light of two or more colors (or wavelengths), and light emitting diodes are inherently narrow-band emitters. No single light emitting diode junction has been developed that can produce white light. A representative example of a white LED lamp or LED package includes a blue LED chip (e.g., made of InGaN and/or GaN), coated with a broad-band emitter, such as phosphor (typically YAG:Ce or BOSE). A broad-band emitter generally has an emission pattern with an approximate full width half maximum (FWHM) greater than 100 nm. Blue LEDs made from InGaN exhibit high efficiency (e.g., external quantum efficiency as high as 70%). In a blue LED/yellow phosphor lamp, a blue LED chip may produce an emission with a wavelength of about 450 nm, and the phosphor may produce yellow fluorescence with a peak wavelength of about 550 nm upon receipt of the blue emission. Part of the blue ray emitted from the blue LED chip passes through the phosphor, while another portion of the blue ray is absorbed by the phosphor, which becomes excited and emits a yellow ray. The viewer perceives an emitted mixture of blue and yellow light (sometimes termed ‘blue shifted yellow’ or ‘BSY’ light) as cool white light. A BSY device typically exhibits good efficacy but only medium CRI Ra (e.g., between 60 and 75), or very good CRI Ra and low efficacy. Cool white LEDs have a color temperature of approximately 5,000 K, which is generally not visually comfortable for general illumination, but may be desirable for the illumination of commercial goods or advertising and printed materials.
Various methods exist to enhance cool white light to increase its warmth. Acceptable color temperatures for indoor use are typically in a range of from 2,700-3,750 K; however, warm white LED devices may be on the order of only half as efficient as cool white LED devices. To promote warm white colors, an orange phosphor or a combination of a red phosphor (e.g., CaAlSiN3 (‘CASN’) based phosphor) and yellow phosphor (e.g., Ce:YAG or YAG:Ce) can be used in conjunction with a blue LED. Cool white emissions from a BSY element (including a blue emitter and yellow phosphor) may also be supplemented with a red LED (with such combination being referred to hereinafter as “BSY+R”), such as disclosed by U.S. Pat. No. 7,095,056 to Vitta, et al. and U.S. Pat. No. 7,213,940 to Negley, et al., to provide warmer light. While such devices permit the correlative color temperature (CCT) to be changed, the CRI of such devices may be reduced at elevated color temperatures.
As an alternative to stimulating a yellow phosphor with a blue LED, another method for generating white emissions involves combined use of red, green, and blue (“RGB”) light emitting diodes in a single package. The combined spectral output of the red, green, and blue emitters may be perceived by a user as white light. Each “pure color” red, green, and blue diode typically has a full-width half-maximum (FWHM) wavelength range from about 15 nm to about 30 nm. Due to the narrow FWHM values of these LEDs (particularly the green and red LEDs), aggregate emissions from the red, green, and blue LEDs exhibit very low color rendering in general illumination applications. Moreover, use of AlInGaP-based red LEDs in conjunction with nitride-based blue and/or green LEDs entails color stability issues, since the efficacy of red LEDs declines more substantially at elevated operating temperatures than does the efficacy of blue and green LEDs.
Another example of a known white LED lamp includes one or more ultraviolet (UV)-based LEDs combined with red, green, and blue phosphors. Such lamps typically provide reasonably high color rendering, but exhibit low efficacy due to substantial Stokes losses.
The highest efficiency LEDs today are blue LEDs made from InGaN. Commercially available devices have external quantum efficiency (EQE) as great as 60%. The highest efficiency phosphors suitable for LEDs today are YAG:Ce and BOSE phosphor with a peak emission around 555 nm. YAG:Ce has a quantum efficiency of >90% and is an extremely robust and well-tested phosphor. White LED lamps made with InGaN-based blue LEDs and YAG:Ce phosphors typically have a CRI Ra of between 70 and 80.
It would also be desirable to provide improved color rendering (e.g., warm white) lighting devices with improved efficacy, with improved color stability at high flux, and/or with longer duration of service. It has also been proposed to produce LEDs of varying colors by the use of narrow-band emitters (instead of phosphors), such as quantum dots. Narrow-band emitters are generally those emitters, which have an approximate full width half maximum (FWHM) less than 100 nm.
A quantum dot (semiconductor nanocrystallites) is a material, generally semiconductor material, having a crystalline structure only a few nanometers in size, and typically includes about a few hundred atoms to about a few thousand atoms. Because of their small size, quantum dots display unique optical and electrical properties that are different in character to those of the corresponding bulk material. The most immediately apparent of these is the emission of photons under excitation, which are visible to the human eye as light. Quantum dots absorb and emit light at wavelengths determined by their size. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of quantum dots shift to the blue (higher energies) as the size of the dots gets smaller and shift to the red as the size of the dots increase. It has been found that a CdSe quantum dot, for example, can emit light in any monochromatic, visible color, where the particular color characteristic of that dot is dependent only on its size. Dots can even be tuned beyond visible light, into the infra-red or into the ultra-violet. The light emitting characteristics of the quantum dot can be adjusted by controlling the size and composition of the quantum dot, and therefore the quantum dot may be employed in various light emitting devices. However, quantum dots may lack stability, at least in part due to their small size, in some environments, impacted by heat and oxidation.